Current generator and method of operating

ABSTRACT

A current generator includes an amplifier having a first terminal configured to receive an input voltage, at least one tunable resistor coupled to a second terminal of the amplifier, a resistor calibration circuit coupled to the at least one tunable resistor, and at least one transistor. A gate of the at least one transistor is coupled to an output of the amplifier, and a terminal of the at least one transistor is coupled to the at least one tunable resistor or a load. The resistor calibration circuit is configured to adjust a resistance setting of the at least one tunable resistor to control a current level of the current generator based on a power supply voltage or a current of a reference resistor.

PRIORITY CLAIM

The present application is a continuation of U.S. application Ser. No. 12/976,504, filed Dec. 22, 2010, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates generally to an integrated circuit, and more particularly to a current generator.

BACKGROUND

In a memory, e.g., a Static Random Access Memory (SRAM), Process Voltage Temperature (PVT) variations may lead to leakage current variations. A current generator in the memory needs to meet a specified current range for the memory, but some current generators cannot maintain the current within the specification due to PVT variations, e.g., when the power supply voltage VDD is high and a gain of an op-amp in the current generator is low.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram showing an exemplary current generator circuit according to some embodiments;

FIG. 2 is a schematic diagram showing an exemplary tunable resistor circuit for the current generator circuit in FIG. 1 according to some embodiments;

FIG. 3 is a schematic diagram showing another exemplary tunable resistor circuit for the current generator circuit in FIG. 1 according to some embodiments; and

FIG. 4 is a flowchart of a method for the exemplary current generator circuit in FIG. 1 according to some embodiments.

DETAILED DESCRIPTION

The making and using of various embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use, and do not limit the scope of the disclosure.

FIG. 1 is a schematic diagram showing an exemplary current generator circuit 102 according to some embodiments. The current generator circuit 102 includes an op-amp 104, a resistance selection circuit 106, and power (PMOS) transistors P1, P2, . . . , and Pn. The input voltage Vtune coupled to a negative terminal of the op-amp 104 is set to control a bias current I_b1 through the resistance selection circuit 106. The positive terminal of the op-amp 104 is coupled to the resistance selection circuit 106. The resistance value of the resistance selection circuit 106 is adjusted to provide the desired current I_b1 for a given Vtune. The resistance selection circuit 106 includes tunable resistor sections 105 coupled in series to provide a target current range for I_b1, e.g., 2-6 μA.

For example, one or more of the switches SW in the resistance selection circuit 106 is closed (or opened) in accordance with Vtune to select the respective one or more tunable resistor sections 105 that provide a desired resistance value corresponding to the target current range for I_b1. The selected resistor section(s) 105 is/are further tunable to vary its resistance value in response to, e.g., power supply variations, as will be described herein below. Thus, by providing resistance selection circuit 106 in which resistor sections 105 are not only selectable (e.g., by respective switches SW and in accordance with Vtune) but also tunable (e.g., by a tunable resistor circuit 200/300 as will be described herein below and in response to power/PVT variations), stable output currents (e.g., I_b1, I_b2, . . . , I_bn) can be obtained while suppressing undesired effects caused by the power/PVT variations.

The power transistors P1, P2, . . . , and Pn are configured to be operating in a saturation mode. For example, the gate voltage Vg of the power transistor P1 is lower than a power supply voltage (VDD) minus a threshold voltage (Vth) of the power transistor P1. The drain voltage Vd of the power transistor P1 is lower than a certain voltage so that the source-drain voltage drop V_(DS) is higher than the source-gate voltage drop V_(GS) minus Vth for a saturation mode. The design (e.g., size) of the power transistors (e.g., P1, P2, . . . , and Pn) and resistance values of tunable resistor sections 105 depend on the specific load 108 (i.e., specific applications) and the desired current range of I_b1. Because the power transistors P1, P2, . . . , and Pn are operating in the saturation mode, the currents I_b1, I_b2, . . . , I_bn can be controlled at a relatively stable value. In particular, if the power transistors P1, P2, . . . , and Pn have similar characteristics (e.g., similar I-V curves), the currents I_b1, I_b2, . . . , I_bn will be similar, because they share the same gate-source voltage V_(GS).

FIG. 2 is a schematic diagram showing an exemplary tunable resistor circuit for the current generator circuit in FIG. 1 according to some embodiments. The tunable resistor circuit 200 includes an exemplary tunable resistor section 105 a for the tunable resistor section 105 in FIG. 1. For example, the tunable resistor circuit 200 includes a resistor calibration circuit 202 that is coupled to multiple tunable resistor sections 105 a in the resistance selection circuit 106. Each tunable resistor section 105 a has the same structure as the resistor section 105 a between nodes A and B shown in more detail.

A comparator 204 in the resistor calibration circuit 202 receives a reference voltage Vref. The power supply voltage VDD is divided by resistors R1 and R2 to provide a portion of VDD, i.e., Vc, to the comparator 204. The comparator determines whether Vc is higher or lower than Vref by comparing Vc and Vref.

If Vc is higher than Vref (i.e., if VDD is higher than expected), the comparator 204 provides a logical 1 to the inverter 206 and HV. The logical 1 at HV turns on an NMOS transistor N2. The inverter 206 provides a logical 0 to HVB to turn off the NMOS transistor N1. As a result, the resistance between nodes A and B is the sum of resistance R3 and R4 that are now connected in series. The NMOS transistors N1 and N2 function as switches for different resistors coupled in parallel (in this example, no resistor for NMOS transistor N1 and a resistor R4 for NMOS transistor N2).

On the other hand, if Vc is lower than Vref (i.e., if VDD is not higher than expected), the comparator 204 provides a logical 0 to the inverter 206 and HV. The logical 0 at HV turns off the NMOS transistor N2. The inverter 206 provides a logical 1 to HVB to turn on the NMOS transistor N1. As a result, the resistance between nodes A and B is the resistance R3 only.

The signals HVB and HV are coupled to all tunable resistor sections 105 a. Thus, the tunable resistor circuit 200 controls each resistor section 105 a to have a higher resistance when VDD is higher. By providing a higher resistance for a higher VDD, there is no need to decrease Vtune in FIG. 1 when the resistance selection circuit 106 has a lower than expected resistance due to PVT variations. Even though two signals, e.g., HVB and HV, are used in this example, any number of control signals or control signal bits can be used for the control of various numbers of resistors.

Without the tunable resistor circuit 200, Vtune may need to be decreased for a lower resistance of the resistance selection circuit 106 due to PVT variations, in order to try to maintain I_b1 at a specified value. This in turn can cause a lower gain of the op-amp 104 under higher VDD, because some device in the op-amp 104 can operate at a linear mode (instead of a saturated mode) due to the higher VDD and lower Vtune. The lower gain of the op-amp 104 can cause the current I_b1 to deviate from the specified current range. For example, the I_b1 can be higher because of the lower gain at higher VDD. In one example, a current generator circuit without the tunable resistor circuit 200, its output current with a setting of 2 μA exhibited 2.73 μA under PVT variations (various process corners at temperature of −40 to 125° C. with a power supply voltage of 0.765-0.935 V). In comparison, for a current generator circuit with the tunable resistor circuit 200, its output current with a setting of 2 μA exhibited 2.28 μA under the same PVT variations.

Therefore, by utilizing the tunable resistor circuit 200 in the current generator circuit 102 in FIG. 1, a relatively stable current/voltage can be specified over a wide range without being significantly affected by PVT variations, because the op-amp 104 operates normally, even under a higher VDD. In one application, the current generator circuit 102 utilizing the tunable resistor circuit 200 can be implemented in a SRAM array to provide a constant current/voltage and thus control the SRAM's leakage current.

FIG. 3 is a schematic diagram showing another exemplary tunable resistor circuit for the current generator circuit in FIG. 1 according to some embodiments. The tunable resistor circuit 300 includes another exemplary tunable resistor section 105 b for tunable resistor sections 105 in FIG. 1. For example, the tunable resistor circuit 300 includes a resistor calibration circuit 302 that is coupled to multiple tunable resistor sections 105 b in the resistance selection circuit 106. Each resistor section 105 b has the same structure as the resistor section 105 b between nodes A and B shown in more detail.

The resistor calibration circuit 302 calibrates the resistance of the resistor section 105 b by utilizing an external reference resistor Rext and an (internal) calibration resistor section 304. (The external reference resistor Rext can be replaced by a relatively reliable internal resistor in another embodiment.) The calibration resistor section 304 has the same structure as the resistor section 105 b. The Rext is coupled to a power transistor P3 and the calibration resistor section 304 is coupled to a power transistor P4. The currents I1 and I2, from Rext and the calibration resistor section 304 respectively, are compared in a comparator 306.

If the Rext is greater than the resistance of the calibration resistor section 304, then I1 is less than I2, and the comparator output is a logical 1 that enables a counter 308 to count up. The counter 308 is programmed to supply a control signal, e.g., s<3:0>. The control signal s<3:0> is supplied to NMOS transistors N3, N4, N5, and N6, as s<0>, s<1>, s<2>, and s<3>, respectively. In one embodiment, the control signal s<3:0> has only one bit that has a logical 1, turning on only one NMOS transistor among NMOS transistors N3, N4, N5, and N6, thus connecting one resistor value to R5 (in series) among different resistance values, e.g., no resistance, R6, R7, or R8 (in order of increasing resistance value).

The value of s<3:0> is increased until the current I1 is greater than the current I2. Then the comparator 306 supplies a logical 0 output that disables the counter 308, thus the counter 308 stops counting. The final s<3:0> value is used as the resistor calibration result and supplied to all tunable resistor sections 105 b. Because the calibration resistor section 304 is located physically close to the tunable resistor sections 105 b (e.g., in the same IC package or on the same PCB), the calibration resistor section 304 can closely follow the local PVT variations and calibrate resistor values accordingly. Even though a 4-bit control signal, e.g., s<3:0>, is used in this example, any number of bits can be used as the control signal for various numbers of resistors.

FIG. 4 is a flowchart of a method for the exemplary current generator circuit in FIG. 1 according to some embodiments. At step 402, an input voltage, e.g., Vtune, is received. At step 404, one or more from at least one tunable resistor, e.g., 105, 105 a, or 105 b, is selected by a resistance selection circuit, e.g., 106. At step 406, a power supply voltage, e.g., VDD in FIG. 2, or a current of a reference resistor, e.g., I1 in FIG. 3, is monitored. At step 408, a resistance setting of the at least one tunable resistor is adjusted. At step 410, an output current, e.g., I_b1, I_b2, . . . , I_bn, is supplied utilizing an op-amp, e.g., 104, based on the input voltage, e.g., Vtune.

In various embodiments, a gate of at least one power transistor, e.g., P1, P2, . . . , Pn, is coupled to an output of the op-amp. A drain of the at least one power transistor is coupled to the at least one tunable resistor, e.g., 105, or a load, e.g., 108. A negative terminal of the op-amp, e.g., 104, is coupled to an input voltage, e.g., Vtune. A positive terminal of the op-amp, e.g., 104, is coupled to the at least one tunable resistor, e.g., 105. A node of the at least one tunable resistor is selected for coupling from the positive terminal of the op-amp to the at least one tunable resistor, e.g., using a resistance selection circuit 106. The at least one tunable resistor is controlled by a resistor calibration circuit, e.g., 202 or 302, coupled to the at least one tunable resistor.

In various embodiments, monitoring a power supply voltage is performed by comparing at least a portion of the power supply voltage and a reference voltage, e.g., Vref. In other embodiments, the current of the reference resistor, e.g., Rext, is compared with a current flowing in an internal calibration resistor section, e.g., 304. The internal calibration resistor section, e.g., 304, has the same structure as one tunable resistor, e.g., 105 b, of the at least one tunable resistor. Adjusting at least one tunable resistor includes selecting one resistor value among multiple resistor values in each of the at least one tunable resistor.

According to some embodiments, a current generator comprises an amplifier having a first terminal configured to receive an input voltage, at least one tunable resistor coupled to a second terminal of the amplifier, a resistor calibration circuit coupled to the at least one tunable resistor, and at least one transistor. A gate of the at least one transistor is coupled to an output of the amplifier, and a terminal of the at least one transistor is coupled to the at least one tunable resistor or a load. The resistor calibration circuit is configured to adjust a resistance setting of the at least one tunable resistor to control a current level of the current generator based on a power supply voltage or a current of a reference resistor.

In a method of operating a current generator in accordance with, an input voltage is received and one or more resistance sections are selected among a plurality of resistance sections in accordance with the input voltage. An output current is outputted in accordance with the selected one or more resistance sections. A resistance value of each of the selected one or more resistance sections is adjusted in response to a variation in a parameter of the current generator.

In some embodiments, a current generator comprises an operational amplifier having a negative terminal configured to receive an input voltage, at least one tunable resistor coupled to a positive terminal of the amplifier, at least one power transistor, and a resistor calibration circuit coupled to the at least one tunable resistor. A gate of the at least one power transistor is coupled to an output of the operational amplifier, and a drain of the at least one power transistor is coupled to the at least one tunable resistor. The resistor calibration circuit is configured to adjust a resistance setting of the at least one tunable resistor to control a current level of the current generator based on a parameter of the current generator.

A skilled person in the art will appreciate that there can be many embodiment variations of this disclosure. Although the embodiments and their features have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosed embodiments, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure.

The above method embodiment shows exemplary steps, but they are not necessarily required to be performed in the order shown. Steps may be added, replaced, changed order, and/or eliminated as appropriate, in accordance with the spirit and scope of embodiment of the disclosure. Embodiments that combine different claims and/or different embodiments are within scope of the disclosure and will be apparent to those skilled in the art after reviewing this disclosure. 

What is claimed is:
 1. A current generator, comprising: an amplifier having a first terminal configured to receive an input voltage; at least one tunable resistor coupled to a second terminal of the amplifier; a resistor calibration circuit coupled to the at least one tunable resistor; at least one first transistor, wherein a gate of the at least one first transistor is coupled to an output of the amplifier, and a terminal of the at least one first transistor is coupled to the at least one tunable resistor or a load; and at least one second transistor, wherein a gate of the at least one second transistor is configured to receive an output current and a drain of the at least one second transistor is coupled to a plurality of resistance sections, wherein the resistor calibration circuit is configured to adjust a resistance setting of the at least one tunable resistor to control a current level of the current generator based on a power supply voltage or a current of a reference resistor.
 2. The current generator of claim 1, wherein the resistor calibration circuit is configured to detect at least a portion of the power supply voltage.
 3. The current generator of claim 2, wherein the resistor calibration circuit comprises a comparator having a first input coupled to receive the detected portion of the power supply voltage and a second input coupled to receive a reference voltage.
 4. The current generator of claim 1, wherein the resistor calibration circuit is configured to detect a current flowing in the at least one tunable resistor.
 5. The current generator of claim 1, wherein the resistor calibration circuit comprises: a calibration resistor section having a structure that is the same as the at least one tunable resistor, and a comparator having a first input coupled to the calibration resistor section, and a second input coupled to the reference resistor having a target resistance value.
 6. The current generator of claim 1, wherein the at least one tunable resistor comprises a plurality of tunable resistors coupled in parallel, and a plurality of switches coupled to the plurality of corresponding tunable resistors.
 7. The current generator of claim 6, wherein the resistor calibration circuit further comprises a counter configured to generate an output to enable at least one switch among the plurality of switches.
 8. A method of operating a current generator, the method comprising: receiving an input voltage; selecting one or more resistance sections among a plurality of resistance sections in accordance with the input voltage; outputting an output current in accordance with the selected one or more resistance sections to a gate of both a first transistor and a second transistor, wherein a drain of the second transistor is coupled to the plurality of resistance sections; and adjusting a resistance value of each of the selected one or more resistance sections in response to a variation in a parameter of the current generator.
 9. The method of claim 8, wherein the parameter comprises a power supply voltage of the current generator.
 10. The method of claim 9, further comprising: determining the variation in the power supply voltage by comparing at least a portion of the power supply voltage and a reference voltage.
 11. The method of claim 9, further comprising: determining the variation in the power supply voltage by comparing a current flowing through a reference resistor and a current flowing through a resistor calibration section.
 12. The method of claim 11, further comprising: adjusting a resistance value of the resistor calibration section in response to a comparison result of the current flowing through the reference resistor and the current flowing through the resistor calibration section.
 13. The method of claim 12, wherein the calibration resistor section has a structure that is the same as one of the plurality of resistance sections.
 14. The method of claim 8, further comprising: supplying the input voltage to a first input terminal of an amplifier, the amplifier further having a second input terminal coupled to the selected one or more resistance sections, wherein, in the outputting, the output current is outputted from an output terminal of the amplifier.
 15. The method of claim 8, further comprising: supplying the output current to a gate of a first transistor, the first transistor further having a drain coupled to a load to be driven.
 16. A current generator, comprising: an operational amplifier having a negative terminal configured to receive an input voltage; at least one tunable resistor coupled to a positive terminal of the amplifier; at least one power transistor, wherein a gate of the at least one power transistor is coupled to an output of the operational amplifier, and a drain of the at least one power transistor is coupled to the at least one tunable resistor; a resistor calibration circuit coupled to the at least one tunable resistor, wherein the resistor calibration circuit is configured to adjust a resistance setting of the at least one tunable resistor to control a current level of the current generator based on a parameter of the current generator; and the resistor calibration circuit further comprising a comparator having a first input coupled to the resistor calibration circuit to receive a detected portion of a power supply voltage, a second input coupled to a reference resistor to receive a reference voltage, and an output coupled to a counter.
 17. The current generator of claim 16, wherein the at least one tunable resistor comprises a plurality of tunable resistors coupled in parallel, and a plurality of switches coupled to the plurality of corresponding tunable resistors.
 18. The current generator of claim 17, wherein the parameter comprises a power supply voltage, the resistor calibration circuit is configured to detect at least a portion of the power supply voltage, and the comparator having a first input coupled to receive the detected portion of the power supply voltage, a second input coupled to receive a reference voltage, and an output coupled to the plurality of switches.
 19. The current generator of claim 17, wherein the resistor calibration circuit comprises: a calibration resistor section having a structure that is the same as one of the plurality of tunable resistors, wherein the counter is configured to adjust resistance settings of the calibration resistor section and the plurality of tunable resistors.
 20. The current generator of claim 16, further comprising at least one other power transistor, wherein a gate of the at least one other power transistor is configured to receive an output current, and a drain of the at least one other power transistor is coupled to a plurality of resistance sections. 